Chemometrics, health risk assessment and probable sources of soluble total petroleum hydrocarbons in atmospheric rainwater, Rivers State, Nigeria

Total petroleum hydrocarbons (TPHs)—(aliphatic and aromatic) were analysed for in atmospheric rainwater between April–June; July–August; September–October depicting early, mid, late rain of 2019. Sampling at Rumuodomaya/Rumuodome and Ogale in Rivers State using basins fastened to a Table 2M above ground and 120 M from high features, Rainwater was analysed after treatment using Agilent GC-FID. Results show cumulative TPHs at R/R were 56.6551 mg/L, 39.5201 mg/L and 7.2283 mg/L, Ogale: 9.1217 mg/L, 59.4923 mg/L and 21.9825 mg/L. Aliphatic hydrocarbons: C5–C8 were < 1, low contamination, other carbon aggregates (C9–C16, C17–C35, and C36–C40) indicate high contamination. Chemometric assessment showed high contamination. TPHs aggregates at Rumuodomaya/Rumuodome were–C8–C11 (1.034 and 1.005) early rain, C18–C25 and C26–C33 has Carbon preference index of 1.287 and 1.630 (mid-rain), C26–C33 has CPI of 1.288 (late-rain), Ogale area, C26–C33 has CPI of 1.732 (early-rain), mid-rain C8–C11 (2.768) and C12–C17 (5.368). Pristane/phytane ratio indicated biogenic and pyrogenic sources. Average carbon length of TPHs for odd n-alkanes were C9–C11 (9.446) and C35–C39 (38.980), C9–C11(10.238), C35–C39 (36.510); C9–C11 (10.240) and C35–C39 (36.934). Average daily intake depicted possible health issues for children and adults as hazard index > 1 for aromatics.

The Pollution Load Index (PLI) is given as: where CF 1 = contamination factor of each TPH aggregate assessed, n = number of components assessed.
Principal component assessment. Principal component assessment (PCA) is a statistical tool used to identify various matrices to derive salient information about components. PCA is a data reduction technique used to find the linear combinations of variables (TPHs) leading to the formation of factors 21 , gives un-rotated and rotated varimax, as rotated varimax is commonly utilized because it facilitates the best interpretation from sets of variables. The rotation is simply a process that allows a new axis to be chosen while maintaining individual components 22 . They are graded as strong (> 0.75), moderate (0.75-0.50), and weak (0.50-0.30) as done by Liu et al. 23 . PCA was determined using Microsoft Excel with XL-STAT add-ins, 2019.
Petroleum hydrocarbon identification using molecular makers. Carbon preference index. Carbon preference index (CPI) is the total of odd n-alkanes divided by the sum total of even n-alkanes for a range of C 8 -C 40 to estimate the relative source identification of either natural or artificial contribution to petroleum hydrocarbon.
CPI is used as a forensic indicator for petroleum source identification of TPHs. CPI values greater than 1 shows the natural contribution from biological phytoplankton plants while CPI less than 1 shows the anthropogenic (artificial) source from ubiquitous contribution 24,25 . TPHs were aggregated into five forms to conduct CPI evaluation: C 8 -C 11 , C 12 -C 17 , C 18 -C 25 , C 26 -C 33, and C 34 -C 40 .
Average carbon length (ACL). Average carbon length is a guide used to evaluate odd n-alkanes dominance per molecule in environmental samples to form link to petrogenic plant source as a forensic tool to assess hydrocarbon contamination 26 . ACL was aggregated into five categories used in CPI assessment and calculated using Eq. (4) below The formula, as shown above, was conducted for other aggregates respectively. ACL values are usually constant in unpolluted rainwater samples but fluctuate as values deplete in polluted rainwater 27 . ACL was assessed for even n-alkanes in the environment to assess the anthropogenic impact in the environment.
Long-chain hydrocarbon/short-chain hydrocarbons (LHC/SHC). Long-chain hydrocarbons are n-alkanes above n-alkanes at C 26 usually from vascular plant-based anthropogenic sources, while short-chain hydrocarbons are n-alkanes below n-alkanes at C26 from phytoplankton or algae-based sources 28 .

Low molecular weight/ high molecular weight (L/H).
It is the ratio of low molecular weight n-alkanes below C20, while above C20 gives the high molecular weight used to determine the n-alkanes. L/H values close to unity (1) show natural input from marine and terrestrial biological sources, while above one (1) indicates inputs from petroleum sources 29 . 19 ratio. It is used as source identification and differentiation for TPHs in rainwater. C31 is assumed to be from terrestrial biogenic hydrocarbons, while C19 proposes marine inputs. The ratio below 0.4 reveals marine sources, while above 0.4 is from terrestrial biogenic or non-marine sources 30 .
Pristane/phytane ratio. It is the ratio of the abundance of pristane to phytane for redox conditions in the aquatic environment, based on the assumption that pristane is from oxygenated (aerobic) source degradation of planktons, while phytane is from reduction (anaerobic) source degradation of planktons by microbes. As such, Pristane/phytane ratio below unity (1) indicates oxic condition (pyrogenic source), while greater than 1 indicates anoxic (biogenic source) 31 .
Human health risk assessment. Human health risk assessment (HHRA) is a tool used to estimate if common and potential contaminants released into the environment will adversely affect the health of humans over a long period from diverse exposure paths (inhalation, injection, dermal). According to Bharadwaj and Machibroda 32 , HHRA may not prove that diseases are connected to an exposure pathway or a particular chemical agent as humans are exposed to numerous chemical agents. According to USEPA 33 , it is based on characterization, risk assessment, hazard identification, receptor characterization, exposure assessment, and risk characterization.
In this study, non-carcinogenic HHRA were evaluated for adults and children for TPHs exposure via dermal and ingestion using USEPA models as shown in Eq. 6 and 7 8,33 .

Discussion
Level of total petroleum hydrocarbons (TPHs) in ambient atmospheric rainwater. Rivers State of Nigeria over the years has been embroiled in numerous cases of crude oil drillings, spills and illicit pipeline bunkering, crude oil, its fractional derivatives and gas pipeline vandalization, massive gas flares, hydrocarbon installation fire outbreaks, refinery operations, these activities are known to release toxic gaseous hydrocarbons into the atmosphere 35 , related emissions from associated-petroleum industries include automobiles, coastal  www.nature.com/scientificreports/ marine transportation, homes, and manufacturing industries utilizing diesel generators, burn-pits also account for total petroleum hydrocarbon (TPH) concentration in the atmosphere [36][37][38] . All the aforementioned releases in the atmosphere undergo chemical interactions, several induced or natural chemical processes such as volatilization, photo-oxidation, and biodegradation, which combine with air moisture before atmospheric rainfall deposition impacting soil and surface water quality 39,40 . The surface water of the present study area is known to be laden with conjugated and straight-chain hydrocarbon 17,41 , most of which come from atmospheric deposition via air and rainfall. Results showed that ambient rainwater at Rumuodomaya/Rumuodome (R/R) and Ogale (O/E) had elevated levels of total petroleum hydrocarbons in their combined forms (aliphatic and aromatic petroleum hydrocarbons) from early rain to late rain ( Table 3). The TPH concentration at Rumuodomaya/Rumuodome decreased from early rain to late rain, while Ogale had a high concentration at mid rain, least for late rain that reveals the rainwater were relatively contaminated and unsafe for human consumption from crude oil and gas processing releases into the atmosphere and subsequently as rainwater. Using USEPA 42 , and TPHCWG 19 , petroleum hydrocarbons categories, there was the presence of both light and heavy carbon chains (C8-C40). C19-C40 indicates the presence of fuel oils; other carbon groups are due to anthropogenic activities. The highest TPH concentrations was predominant for C40 across all locations and sampling regimens and shows that higher molecular weight hydrocarbons were present. Specific range from Table 3 as carbon chain length shows TPHs value of 0.0046 of C21-TPHs (56.6551) of C40 (early rain); 0.0683 of C8-TPHs of 6.5533 of C40 (mid rain) and 0.0142 of C34-1.6351 (late rain) at Rumuodomaya/ Rumuodome (R/R) while at Ogale (O/E), it ranged from 0.0500 of C19-1.8601 of C40 (early rain), 0.0342 of C8-22.237 (mid rain) and 0.0050 of C37-1.511 (late rain). A look at Table 3 shows that variation exist between sampling period and study sites. The likely reason may be because of transboundary movement of petroleum hydrocarbons particulates that were in the atmosphere and closeness to source and intensity of emission. Also, the study area lies on long coastal area of the Atlantic Ocean, at early rain, there may be high concentration of gaseous pollutants, as intensity and volume of rain increases, dilution effect can occur. Again, since it is a coastal region of Atlantic Ocean, the humidity may be above average throughout the year and may cause local variation. Similar studies by Ali et al. 10 showed the presence of TPHs in River water in significant levels in three-sampling stations as they got lower concentration in October 2018, while January 2019 had a higher concentration in Al-Gharraf, Iraq, which was attributed to emission from sewage releases into rivers and municipal wastes from cities and farmland but the levels obtained were lower in comparison to the TPHs values presented here. According to Kennedy et al. 3 , rainfall reveals the overall and regional atmospheric Table 9. Average daily intake of TPHs exposure in rainwater for adults. www.nature.com/scientificreports/ quality inclusive of all forms of emissions to the atmosphere, as similar assessment by Griffiths and Timperley 43 noted the presence of visible oil sheens on vehicle screens with particulate matter. Rainfall intensity and duration known to vary in pattern, may have influence as continuous rain events takes place, the concentration reduced continuously from high to low and vice versa due to the seasonal pattern (wet and dry) peculiar with Nigeria, in addition, the influence of human activities such as helicopter, aeroplane fuel discharge on air transit, anaerobic microbial events in waste dumps and marine organisms, flue gas burning, marine vessels and transports, pyrolysis and petrogenic events that are transboundary from offshore (Atlantic ocean) to onshore (land) from wind movement (South to Northwest and Eastward) may influence petroleum hydrocarbons concentration.

Chemometric assessment. Contamination factor and pollution load index. Contamination factor (CF)
and pollution load index (PLI) were conducted on aliphatic and aromatic hydrocarbons within THGCWG 19 standards as shown in Table 4, aliphatic hydrocarbons via both locations, C5-C8 were < 1, implying low contamination, while other carbon aggregates (C9-C16, C17-C35 and C36-C40) indicates high contamination except for early and late rain of C17-C35 and C36-C40 at Rumuodomaya/Rumuodome (R/R), also early and late rain of C36-C40 at Ogale (O/E) respectively. Considering aromatic hydrocarbons, C5-C8 at the R/R location were below one (1) while that of O/E early rain was considered contaminated, other carbon aggregates were highly present. Using PLI assignment, aliphatic hydrocarbons at R/R early rain were within safe background level except for mid and late rain that were above one (1), indicating high pollution of atmospheric rainwater, at Ogale (O/E), rainwater was highly polluted for all three rain sampling periods but aromatic hydrocarbons at both R/R and O/E were high and polluted as they were above one (1) 44 . The contamination of water by TPH is associated with increase in particulate matter of fine particle size, high temperature, reduced dissolved oxygen, low salinity, and anaerobic reactions 45 . In addition, TPHs at higher carbon ranges are known to form oily films that prevent limited sunlight penetration thus influencing poor water quality leading to taste and odour issues over time.
Principal component analysis. Total petroleum hydrocarbons (TPHs) were subjected to principal component analysis using rotated varimax as seen in Table 5. Principal component analysis is a statistical tool used to aggregate data set to a linear regression (y = mx + C) between 0.00-0.99, which is computed using Liu et al. 23  www.nature.com/scientificreports/ were positive and negative TPHs components across the two factors assigned. Using Liu et al. 23 at R/R, Factor 1 had moderate and strong regression grades except for C37 and C40, which had weak regression, while Factor 2 had moderate and strong regression, at O/E Factor 1 had predominant weak and strong regression across all TPHs components assessed except C31, while Factor 2 had strong, moderate and weak regression except for C16-C27 and C37-C40 that were very weak (< 0.30). As we can infer across both locations (R/R and O/E), positive components are due to the presence of the TPH components from varying sources either petrogenic or phytogenic petroleum sources. The negative components can be due to atmospheric reactions with pyrogenic petroleum sources (combustion of carbonaceous substances), marine sea sprays, and climatic conditions 46 .

Petroleum hydrocarbon source identification. Carbon preference index. Carbon preference index
(CPI) shown in Table 6, indicated that values above one (1) were due to natural sources (terrestrial vascular floras) as compared to CPI below one (1), more in number that was due to anthropogenic and petroleum activities from the combustion of crude oil, gas flares, refinery and petrochemical plant, emission of organics from industries as a major economic activity within the area. The TPHs aggregates at Rumuodomaya/ Rumuodome (R/R) were highest at C8-C11 (1.034 and 1.005) early rain. C18-C25 and C26-C33 have CPI of 1.287 and 1.630 (mid-rain). C26-C33 has a CPI of 1.288 (late-rain) but at Ogale area, C26-C33 has the highest CPI value of 1.732 (early-rain), the mid-rain highest value is for C8-C11 (2.768) and C12 -C17 (5.368) while late-rain exhibited a CPI value of 1.490, 1.592, 1.193 and 1.728 (C8-C11, C12-C17, C18-C25 and C26-C39), all these were greater than CPI value of one (1) but accounted for only 40% while values less than one (1) represent 60% hence CPI were dominated by anthropogenic origin of petroleum processing 13,47 .
Average carbon length (ACL). Average carbon length (ACL) values as evaluated (Table 7) fluctuated for early rain as compared to mid rain and late rain and varied slightly for odd n-alkanes, showing there was a little anthropogenic contribution to the odd carbon aggregates as compared to early rain respectively 48 . ACL of TPHs aggregates shows that for odd n-alkanes minimum and maximum values were C9-C11 (9.446) and C35-C39  www.nature.com/scientificreports/ dome (R/R) but for Ogale (O/E) area, the ACL of TPHs aggregates of odd n-alkanes were C9-C11 and C35-C39 (9.631 and 36.647; 9.323 and 37.210; 9.362 and 35.115) for early, mid and late rain. Looking at even n-alkanes, C8-C10 and C34-C40 (8.596 and 39.365; 9.740 and 39.660; 9.240 and 36.727) for early, mid and late rain, again for n-alkanes, similar variation as seen in odd n-alkanes were attributed to slight anthropogenic input 49 . The results of odd ACL vs CPI plots for early rain, mid rain, and late rain reveal that regression (R 2 ) for Rumuodomaya/ Rumuodome (R/R) were 0.4264, 0.0067, 0.0048, while O/E were 0.0103, 0.5062, 0.3358 indicating an increase in ACL as CPI values fluctuate. The regression for even-n-alkanes ACL/CPI for R/R were 0.4882, 0.0389, 0.0266, O/E were 0.0242, 0.5976, 0.4357 signifying similar trends for odd-n-alkanes. This confirms that CPI assessments were influenced by both natural and anthropogenic sources.
Petroleum source diagnostics. Petroleum hydrocarbon source diagnostic ratio were assessed and depicted in Table 8. with an aim to assess possible source across different environmental matrices, floras and faunas. Average carbon length (ACL) (odd n-alkanes) has the highest value of 37.854 (early rain) and 28.248 (late rain) for Rumuodomaya/ Rumuodome (R/R) and Ogale (O/E) sampling sites. ACL (even n-alkanes) has 39.854 and 33.565 for early and mid-rain of both study areas. The two location's long-chain hydrocarbons/short-chain hydrocarbon ratio (LHC)/SHC) of odd n-alkanes and LHC/SHC of even n-alkanes, the highest values were as follows: 22.268, 220.33 (early rain), and 4.019, 4.881 (late rain). C31/C19 showed that all three rainfall events evaluated were above 0.4, thus shows the impact of land sources as an array of emissions from crude oil processing, industrial flue gases, power plants, automobile, heavy-duty vehicles, and flare gases 50 . In addition, elevated temperature in the soil releases volatile organic compounds while decreases in temperature usually at night activate microbial decomposition of organic matter leads to the production of petroleum hydrocarbons, influences the TPH concentration in the atmosphere. Low molecular/high molecular (L/H) weight n-alkanes evaluated showed that the L/H ratio was below one (1), suggesting impact from phytogenic and pyrogenic sources. Long-chain hydrocarbons/short-chain hydrocarbons (LHC/SHC) assessed for odd n-alkanes reveals that at R/R early rain and O/E late rain were above one (1), indicating terrestrial floral sources, while below 1 is due to phytoplankton sources. LHC/SHC (even n-alkanes) were all above one (1), thus confirming all were from (anthropogenic) petroleum sources (Table 8). Cumulative CPI assessed were below one (1) from a pyrogenic source. Cumulative average carbon length (ACL) (odd and even n-alkanes) showed fluctuating values confirming impact from anthropogenic sources. The plot of cumulative odd ACL against cumulative CPI via the influence of the three rain regimens showed that regression, R 2 were 0.9454 and 0.0607 at R/R and O/E, while even ACL versus CPI plot gave regres- www.nature.com/scientificreports/ sion, R 2 = 0.9594 and 0.291 respectively, hence depicting that petroleum hydrocarbon in atmospheric rainwater of the study area may indicate non-marine and anthropogenic sources. The ratio of pristane/phytane exceeded one (1) for R/R early rain and late rain in relation to O/E mid rain indicating biogenic (aerobic) sources, as anaerobic indices are possible for values less than one (1) for R/R mid rain and O/E early and late rains 51 . In addition, different assessment has shown that pristane/phytane less than 0.8 implies saline to the hypersaline condition due to extreme evaporation and carbonaceous deposition, whereas pristane/phytane above 3 is due to oxygenated to non-oxygenated degradation of planktons from anaerobic organisms 52  www.nature.com/scientificreports/ bi-products of chlorophyll in the aquatic environment, pristane are produced from the breakdown of zooplankton and phytoplankton (algae) in an oxygenated aquatic environment to form lipids, whereas phytanes are from anaerobic degradation of aquatic planktons by microbial organisms (cyanobacteria) 53 . Therefore, we can infer that petroleum hydrocarbon in atmospheric rainwater of the study area were from diverse sources (natural and human activities) that have the potential to impact on rainwater quality in relation to environmental releases such as microbial releases due to anaerobic condition in aquatic or waste dump areas, petroleum spills, flue gases, natural gas release into the atmosphere from earth sources and pyrolytic activities.

Risk assessment.
A serious problem of the study area is non availability of potable water especially in the rural communities, even as the surface and underground water of the area are contaminated 16,17,41 , hence rain water becomes aa available alternative. Average daily intake (ADI) of aliphatic and aromatic petroleum hydrocarbons aggregates was assessed for ambient atmospheric rainwater from Rumuodomaya/Rumuodome (R/R) and Ogale (O/E). Two exposure pathways were calculated using ingestion (oral) and dermal (skin contact) to assess the average daily intake (ADI) of rainwater for adults and children as depicted in Tables 9 and 10, but  from Table 9, aliphatic PHs of C33-C40 were exposed to adult through dermal and ingestion by 5.47 × 10 -02 and 5.62 × 10 -02 via consumption of early rain at Rumuodomaya/Rumuodome, the same for aromatic PHs of C33-C40 (5.47 × 10 -2 and 5.62 × 10 -2 ). At Ogale, the same adults are more at risk of aliphatic PHs of C19-C32 and C33-C40 (2.07 × 10 -2 , 2.02 × 10 -2 ; 2.82 × 10 -2 , and 2.74 × 10 -2 ) through ingestion or dermal contact with mid-rain while they are highly exposed to C17-C32 and C33-C40 by 2.50 × 10 -2 ; 2.04 × 10 -2 ; 2.82 × 10 -2 and 2.74 × 10 -2 on contact with mid-rain through ingestion and dermal pathway and to C17-C32 (the same exposure route) risk value of 1.21 × 10 -2 and 1.18 × 10 -2 (late rain). Non-carcinogenic risk assessment for adults and children evaluated having obtained hazard quotient (HQ) for exposed pathways as shown in Tables 11 and 12. Preliminary assessment of ADI and subsequent evaluation of HQ shows that children had elevated values compared to adults. Hazard index (HI) as shown in Fig. 1 for aliphatic petroleum hydrocarbons reveals that at R/R, mid rain for adults and children were above one (1), thus there is an inherent health risk. At O/E, mid rain for children was above one (1), as compared to adults. The hazard index for aromatic petroleum hydrocarbons shown in Fig. 2 shows that all locations including the rain period were above one (1). Risk-based mapping using carbon ranges were proposed by MADEP 54,55 and TPHCWG 56 to assess the health-based risk over a period for all age grade, sex orientation, diet, family traits, lifestyle and current state of health to derive salient points for regulatory consideration and action plan models. The analysed HI values reveals that ingestion of TPHs contaminated rainwater by both adult and children leads to bioaccumulation resulting in disruption of biochemical and physiological activities in the human body causing negative health outcomes usually after a short period 11 .
-0.5 0 0  www.nature.com/scientificreports/ In the atmosphere, where free chloride ions exist in the presence of ultra-violet radiation and high-temperature conditions permit (Table 13), combines with aliphatic petroleum hydrocarbons (aerosols) to form toxic polychlorinated-n -alkanes (PCAs), and polychlorinated biphenyls (PCBs). These subsequently dissolved in rainwater impacting water quality as when ingested over a period of time can cause detrimental health issues such as liver and kidney dysfunctions, dermatitis, dizziness, and severe headaches 17 . In addition, PAHs photochemical reactions form diones, nitro-PAHs, dinitro-PAHs, and other PAHs components known to cause carcinogenic and mutagenic effects and resulting in human health crises such as bone suppression and decreased blood cell production with reproductive complications 57 .
TPHs ingestion via atmospheric rainwater has been associated with headaches, fatigue, nausea, diarrhoea, and irritation of gastrointestinal tracts over a long period 58 . Renal disorder, digestive disruption, and interstitial oedema are also negative health outcomes associated with consumption of contaminated rainwater 59 . Dermal contact with contaminated rainwater sources can cause skin inflammation and rashes, dermatitis, blisters, eye irritation, and degreasing burns where been reported by persons during bathing periods in amazon, Ecuador 60 . In children, the presence of dandruff and psoriasis has been attributed to TPHs 61 . Petroleum hydrocarbon represents all aliphatic and aromatic petroleum formulations. Aliphatic petroleum hydrocarbon is known to cause neurological disorders, cancer of the mouth, stomach, and uterine disorders 60 . Aromatic petroleum hydrocarbons in the form of benzene, toluene, polycyclic aromatic hydrocarbons cause haematological, immunological, neurological, gastrointestinal disorders inclusive of possible death associated with these contaminants. This study forms the basis of call to policy makers in Nigeria on the need to provide potable water to its citizens, especially in the crude oil and gas rich region (Niger Delta area), lack of potable water has forced the inhabitant to depend on surface and rainwater for drinking and domestic needs with its attendant health risks. Unachukwu and coworker 62 reported that non-communicable diseases (NCDs) such as cardiovascular disease, diabetes mellitus, cancer, renal diseases, liver failure and so on, which may be associated with oil pollution are now highly diagnosed and reported in hospitals within the study area and may be more pronounced amongst poor rural dwellers. Having assessed these possible health effects, children are most at risk as reports by Steven 60 , give worrisome reports that need adequate attention to mitigates these issues in the long run.

Conclusion
The present study assessed total petroleum hydrocarbons (TPHs) in rainwater sampled via three regimens (early rain, mid rain, and late rain) at Rumuodomaya/ Rumuodome and Ogale in Rivers State, Nigeria. The TPH concentration at Rumuodomaya/ Rumuodome decreased from early rain to late rain, while Ogale had a high concentration at mid rain, least for late rain that reveals the rainwater were relatively contaminated and unsafe for human consumption due to crude oil and gas processing releases into the atmosphere and subsequently as rainwater. Chemometric assessment using total petroleum hydrocarbon criteria working group standards showed that both aliphatic and aromatic petroleum were relatively high. TPHs source identification showed that carbon preference index and average carbon length gave correlation, which implies that contaminations were due to anthropogenic sources, probably hydrocarbon been the major economic activity in the area. A risk assessment conducted showed that hazard index was above one (1) for aromatic petroleum hydrocarbons compared to aliphatic petroleum hydrocarbon that had varying levels. The pollution levels show that children were more at risk from continuous oral and dermal exposure to TPHs in rainwater. Government agencies with assistance from oil and gas stakeholders can set up a monitoring stations for air, soil, and water contamination sources to assist in the development of a cost-effective approach and remediation action plan to avert possible health issues in the future. www.nature.com/scientificreports/ Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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